Molecular fluorine laser

ABSTRACT

An efficient F 2  laser is provided with improvements in line selection, monitoring capabilities, alignment stabilization, performance at high repetition rates and polarization characteristics. Line selection is preferably provided by a transmission grating or a grism. The grating or grism preferably outcouples the laser beam. The line selection may be fully provided at the front optics module. A monitor grating and an array detector monitor the intensity of the selected (and unselected) lines for line selection control. An energy detector is enclosed in an inert gas purged environment at slight overpressure. A blue or green reference beam is used for F 2  laser beam alignment stabilization and/or spectral monitoring of the output laser beam. The blue or green reference beam advantageously is not reflected out with a atomic fluorine red emission of the laser and is easily resolved from the red emission. The clearing ratio of the laser gas flow through the discharge area is reduced by narrowing the discharge width using improved laser electrodes and/or by increasing the gas flow rate through the discharge while maintaining uniformity by using a more aerodynamic discharge chamber. The F 2  laser beam is substantially polarized, e.g., 98% or better, using at least one intracavity polarization element preferably in combination with Brewster discharge chamber windows.

PRIORITY

This application claims the benefit of priority to U.S. provisionalapplications No. 60/173,993, filed Dec. 30, 1999, and No. 60/170,919,filed Dec. 15, 1999, each application being assigned to the sameassignee as the present application and being hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a molecular fluorine (F₂) laser, andparticularly to an F₂-laser with an improved resonator design andimproved beam monitoring and line-selection for providing stable outputbeam parameters at high operating repetition rates.

2. Discussion of the Related Art

a. VUV microlithography

Semiconductor manufacturers are currently using deep ultraviolet (DUV)lithography tools based on KrF-excimer laser systems operating around248 nm, as well as the following generation of ArF-excimer laser systemsoperating around 193 nm. Vacuum UV (VUV) will use the F₂-laser operatingaround 157 nm.

The construction and electrical excitation of the F₂-laser differsfundamentally from the rare gas-halide excimer lasers mentioned above.The laser gas of a rare gas-halide excimer laser, such as the KrF or ArFlaser, includes a laser active molecular species that has no boundground state, or at most a weakly bound ground state. The laser activegas molecule of the excimer laser dissociates into its constituentatomic components upon optical transition from an upper metastable stateto a lower energy state. In contrast, the laser active gas constituentmolecule (F₂) of the F₂-laser responsible for the emission around 157 nmis bound and stable in the ground state. In this case, the F₂ moleculedoes not dissociate after making its optical transition from the upperto the lower state.

The F₂-laser has an advantageous output emission spectrum including oneor more lines around 157 nm. This short wavelength is advantageous forphotolithography applications because the critical dimension (CD), whichrepresents the smallest resolvable feature size producible usingphotolithography, is proportional to the wavelength. This permitssmaller and faster microprocessors and larger capacity DRAMs in asmaller package. The high photon energy (i.e., 7.9 eV) is also readilyabsorbed in high band gap materials like quartz, synthetic quartz(SiO₂), Teflon (PTFE), and silicone, among others, such that theF₂-laser has great potential in a wide variety of materials processingapplications. It is desired to have an efficient F₂ laser for these andother industrial, commercial and scientific applications.

b. line-selection and line-narrowing

The emission of the F₂-laser includes at least two characteristic linesaround λ₁=157.629 nm and λ₂=157.523 nm. Each line has a naturallinewidth of less than 15 pm (0.015 nm), and in the usual pressure rangebetween 2-4 bar, the natural linewidth can be less than 2 pm. Theintensity ratio between the two lines is I(λ₁)/I(λ₂)0.7. See V. N.Ishenko, S. A. Kochubel, and A. M. Razher, Sov. Journ. QE-16, 5 (1986).FIGS. 1a and 1 b illustrate the two above-described closely-spaced peaksof the F₂-laser spontaneous emission spectrum. FIG. 1b shows a third F₂laser emission line around 157 nm that is observed when neon is used asa buffer gas, but that is not observed when the buffer gas used isstrictly helium, as shown in FIG. 1a (see U.S. patent application Ser.No. 09/317,526, which is assigned to the same assignee as the presentapplication and is hereby incorporated by reference). Either way, thecharacteristic bandwidth of the 157 nm emission of the F₂ laser iseffectively more than 100 pm due to the existence of the multiple lines.

Integrated circuit device technology has entered the sub-0.18 micronregime, thus necessitating very fine photolithographic techniques. Linenarrowing and tuning is required in KrF- and ArF-excimer laser systemsdue to the breadth of their natural emission spectra (around 400 pm).Narrowing of the linewidth is achieved most commonly through the use ofa line-narrowing unit consisting of one or more prisms and a diffractiongrating known as a “Littrow configuration”. However, for an F₂-laseroperating at a wavelength of approximately 157 nm, use of a reflectivediffraction grating is unsatisfactory because a typical reflectivegrating exhibits low reflectivity and a laser employing such a gratinghas a high oscillation threshold at this wavelength. The selection of asingle line of the F₂ laser output emission around 157 nm has beenadvantageously achieved and described at U.S. patent application Ser.Nos. 09/317,695 and 09/317,527, each of which is assigned to the sameassignee as the present application and is hereby incorporated byreference. It is desired to improve upon the line-selection techniquesset forth in the '695 and '527 applications. Moreover, it is desired tohave a way of monitoring the quality of the line selection beingperformed.

For an excimer laser, such as a KrF- or ArF-excimer laser, thecharacteristic emission spectrum may be as broad as 400 pm. To narrowthe output bandwidth, one or more dispersive line-narrowing optics areinserted into the resonator. To increase the angular (and spectral)resolution commonly more than one optical dispersive element isintroduced. A typical line-narrowing arrangement for a KrF- orArF-excimer laser includes a multiple prism beam expander before agrating in Littrow configuration. The length of the resonator increasesas more optical elements as added. Also, additional optical interfacesgives rise to losses which result in a decrease of the output power fora given input voltage. It is desired to reduce the number of opticalcomponents in a line-narrowed excimer laser.

c. absorption

The F₂-laser has been known since around 1977 [see, e.g., Rice et al.,VUV Emissions from Mixtures of F₂ and the Noble Gases-A Molecular F₂laser at 1575 angstroms, Applied Physics Letters, Vol. 31, No. 1, Jul.1, 1977]. However, previous F₂-lasers have been known to exhibitrelatively low gains and short gas lifetimes. Other parameters such asthe pulse-to-pulse stabilities and laser tube lifetimes have beenunsatisfactory. In addition, oxygen and water exhibit high absorptioncross sections around the desired 157 nm emission line of the F₂-laser,further reducing overall efficiency at the wafer when encountered by thelaser beam anywhere along its path. To prevent this absorption, one canmaintain a purged or evacuated beam path for the F₂-laser free ofoxygen, hydrocarbons and water (see U.S. patent application Ser. No.09/343,333, which is assigned to the same assignee as the presentapplication and is hereby incorporated by reference). In short, despitethe desirability of using short emission wavelengths forphotolithography, F₂-lasers have seen very little practical industrialapplication to date. It is desired to have an F₂-laser with enhancedgain, longer pulse lengths, enhanced energy stability, and increasedlifetime.

F₂-lasers are also characterized by relatively high intracavity losses,due to absorption and scattering in gases and optical elements withinthe laser resonator, particularly again in oxygen and water vapor whichabsorb strongly around 157 nm. The short wavelength (157 nm) isresponsible for the high absorption and scattering losses of theF₂-laser, whereas the KrF-excimer laser operating at 248 nm does notexperience losses of such a comparably high degree. In addition, outputbeam characteristics are more sensitive to temperature inducedvariations effecting the production of smaller structureslithographically at 157 nm, than those for longer wavelength lithographysuch as at 248 nm and 193 nm. Therefore, the advisability of takingsteps to optimize resonator efficiency is recognized in the presentinvention.

d. atomic fluorine visible emission

The VUV laser radiation around 157 nm of the F₂-molecule has beenobserved as being accompanied by further laser radiation output in thered region of the visible spectrum, i.e., from 630-780 nm. This visiblelight originates from the excited fluorine atom (atomic transition). Itis desired to have an F₂-laser wherein the output in the visible regionis minimized and also to maximize the energy in the VUV region.

Although the active constituent in the gas mixture of the F₂-laser isfluorine, the amount of pure fluorine amounts to no more than about 5 to10 mbar of partial pressure within the gas mixture, and typically lessthan 5 mbar. A higher overall pressure is needed to sustain a uniformdischarge. Consequently, a buffer gas is needed to raise the dischargevessel pressure, typically to well above atmospheric pressure (e.g.,2-10 bars), in order to achieve an efficient excitation within thedischarge and realize an efficient output of the 157 nm radiation.

For this reason, F₂-lasers have gas mixtures including an inert buffergas which is typically helium and/or neon. When helium is used, however,the output in the red visible region can range from one to more thanthree percent of the VUV emission. In addition, the VUV pulse lengthstend to be relatively short. The visible output of the F₂ laser has beenadvantageously reduced by using neon or a combination of neon and heliumas the buffer gas in the F₂ laser (see the '526 application). Inaddition, the length of the VUV output pulses of the F₂ laser has beenshown in the '526 application to be increased using neon in the gasmixture improving both line selection and line narrowing capability. Itis desired to further reduce the influence of the visible emission onthe performance of the F₂ laser.

e. relatively short pulse duration

As noted above, the pulse duration of the F₂ laser is relatively shortcompared with that of the rare gas-halide excimer lasers. For example,KrF laser pulses make between four and six roundtrips through the laserresonator, whereas F₂ laser pulses typically make only one to tworoundtrips. This reduces the effectiveness of the line-selection andnarrowing components of the resonator. The short pulse duration alsoreduces the polarizing influence of surfaces aligned at Brewster's anglesuch as the windows on the laser tube or of other polarizing componentsin the resonator. The pulse duration is advantageously increased asdescribed in the '526 application using neon in the gas mixture. Acomparison of the F₂ laser emission linewidths in FIG. 1a with thoseshown at FIG. 1b illustrate the effect of increasing the pulse durationby substituting neon for helium in the gas mixture. However, when thelaser tube windows are aligned at Brewster's angle, the output laserbeam is still only about 70% polarized. It is desired to have a F₂ laserwhich emits a substantially polarized beam, e.g., such that the beamexhibits a 98% or greater polarization.

f. beam parameter and alignment monitoring

It is desired that the pulse energy, wavelength and bandwidth of theoutput beam each be stabilized at specified values, particularly forphotolithography lasers. Moreover, it is desired to maintain asubstantially constant energy dose at the workpiece. Further, it isdesired to maintain a steady and predetermined beam alignment. Varioustechniques are known for monitoring the pulse energy and/or other beamparameters and controlling the discharge voltage, the composition ofgases in the laser tube and/or the hardware and optics for stabilizingthese parameters in photolithography lasers (see U.S. patent applicationSer. Nos. 09/447,882, 60/124,785, 60/171,717, 09/418,052, 60/159,525,09/416,344, 09/379,034, 60/127,062, 60/160,126 and 09/513,025, each ofwhich is assigned to the same assignee as the present application and ishereby incorporated by reference. Beam alignment techniques aredescribed at U.S. Pat. No. 6,014,206 and U.S. patent application Ser.No. 09/271,020, which are assigned to the same assignee as the presentapplication, and U.S. Pat. No. 5,373,515, each of which is herebyincorporated by reference. The visible emission of the F₂ laser and thetendency of the VUV emission of the F₂ laser to undergo absorptionpresent some difficulties when applying the techniques particularly setforth in these patents and patent applications. It is therefore desiredto effectively implement beam alignment and parameter monitoringtechniques in a F₂ laser system.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an efficient F₂ laser forindustrial, commercial and scientific applications such asphotolithography and other materials processing applications.

It is also an object of the invention to provide improved line-selectiontechniques for the F₂ laser, and to provide a technique for monitoringthe quality of the line selection being performed.

It is a further object of the invention to provide a F₂ laser havingimproved resonator efficiency.

It is another object of the invention to provide techniques for reducingthe influence of the visible emission on the performance of the F₂laser.

It is also an object of the invention to provide a F₂ laser that emits asubstantially polarized beam, e.g., such that the beam exhibits a 98% orgreater polarization.

It is a further object of the invention to effectively implement beamalignment and parameter monitoring techniques in a F₂ laser system.

It is an additional object of the invention to provide an excimer laser,such as a KrF or ArF excimer laser having an efficient line-narrowingoptical resonator.

In accordance with the above objects, a F₂ laser in accord with thepresent invention includes a laser tube filled with a laser gas mixtureand having a plurality of electrodes connected with a power supplycircuit for energizing the gas mixture. A laser resonator including thetube for generating a 157 nm laser beam includes a line selection unitfor selecting one of multiple closely-spaced characteristic emissionlines around 157 nm.

In a first aspect of the invention, line selection is provided by atransmission diffraction grating. The preferred grating is made of CaF₂and also serves to outcouple the laser beam. The transmission grating inaccord with the first aspect advantageously permits a straight,shortened and more efficient F₂ laser resonator.

In a second aspect of the invention, line selection is provided for a F₂laser by a grism. The preferred grism also serves either to outcouplethe beam or as a highly reflective resonator reflector. The grism inaccord with the second aspect advantageously provides enhanceddispersion and efficiency.

Also in accord with this aspect of the invention, an excimer ormolecular fluorine laser includes a laser output coupler including agrism, which is a combination of a prism and a grating. Such acombination of prism and grating within one element advantageouslyimproves the resolving power of a single dispersive element and reducesthe internal resonator losses by a minimum of optical interfaces.

The grism used directly as an output coupler for an excimer or molecularfluorine laser advantageously combines four different tasks in oneelement: partial reflection (or output coupling) for the resonator,dispersion and line narrowing or line selection, suppression ofbackground radiation, such as amplified spontaneous emission (ASE)radiation (or a parasitic second line), and pointing stabilization ofthe selected line (wavelength). The grating-prism (or grism) may bedesigned in such a manner that it realizes a straight beam path for theselected wavelength which is used as the output beam for use with anapplication process, operating like a common mirror if the blaze angleof the grating is equal to the prism angle for the selected wavelength.Thus, the laser resonator can be very short even if it is containing aline selecting or line narrowing element having the direction of beampropagation in a straight line.

In a third aspect of the invention, line selection for an F₂ laser isfully performed at the front optics module of the laser resonatorresulting in a more efficient resonator.

In a fourth aspect of the invention, a monitor grating and an arraydetector are provided for monitoring and controlling the intensity ofthe selected (and/or unselected) lines and for monitoring the stabilityof the selected wavelength. The quality of the line selection may beadvantageously monitored.

In a fifth aspect of the invention, a F₂ laser system includes an energydetector provided in an enclosure purged with an inert gas at a slight,regulated overpressure. Advantages include reduced turbulence typicallyassociated with high gas flow and a reduced rate of deposition ofcontaminants on optical surfaces. The fifth aspect may be advantageouslycombined with the fourth aspect.

In a sixth aspect of the invention, a blue or green reference beam(e.g., having a wavelength between 400 nm and 600 nm) is used for F₂laser beam wavelength calibration and/or alignment stabilization. Theblue or green reference beam advantageously is not reflected out withthe red emission of the laser and is easily resolved from the redemission.

In a seventh aspect of the invention, the clearing ratio of the lasergas flow through the discharge area of a F₂ laser is improved. Thereduced clearing time is provided by narrowing the discharge width usingimproved laser electrodes and/or by increasing the gas flow rate throughthe discharge while maintaining uniformity by using a more aerodynamicdischarge chamber. F₂ laser operation at higher repetition rates isadvantageously permitted by the reduced clearing ratio in accord withthe seventh aspect of the invention.

In an eighth aspect of the invention, a F₂ laser is provided with asubstantially polarized output beam. The polarization is provided by athin film polarizer, a double reflection prism and/or Brewster windows.The polarization provided by the eighth aspect of the invention isadvantageously 98% or better.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows the emission spectrum of a free-running F₂ laser withhelium as the buffer gas.

FIG. 1b shows the emission spectrum of a free-running F₂ laser with neonas a buffer gas.

FIG. 2 schematically shows a F₂ laser system in accord with the presentinvention.

FIG. 3a schematically shows a F₂ laser resonator including atransmission diffraction grating for line selection in accord with thefirst aspect of the invention.

FIG. 3b schematically shows a F₂ laser resonator including atransmission diffraction grating as an output coupler also in accordwith the first aspect of the invention.

FIG. 3c schematically shows an alternative embodiment of a F₂ laserresonator including a transmission diffraction grating as an outputcoupler also in accord with the first aspect of the invention.

FIG. 4a schematically shows a F₂ laser resonator including a grism forline selection in accord with the second aspect of the invention.

FIG. 4b schematically shows a F₂ laser resonator including a grism as anoutput coupler also in accord with the second aspect of the invention.

FIG. 5 illustrates angular dispersion by an ordinary prism.

FIG. 6a illustrates a grism including a prism with an attached grating.

FIG. 6b illustrates a grism including a prism having a grating etchedinto the prism material.

FIG. 7a shows a reflective grating.

FIG. 7b shows a reflective grism.

FIG. 8 shows a grism designed for straight through selected linepropagation.

FIGS. 9a-9 c and 9 g show alternative line-narrowing resonatorconfigurations.

FIGS. 9d-9 f and 9 h illustrate spectral distributions and backgroundradiation levels of output beams of the resonator configurations ofFIGS. 9a-9 c and 9 g.

FIG. 10a shows a resonator including a grism output coupler.

FIG. 10b illustrates a spectral distribution and zero backgroundradiation level of an output beam of the resonator configuration of FIG.10a.

FIG. 11 schematically shows a F₂ laser resonator having line selectionfully performed at the front optics module of the resonator in accordwith the third aspect of the invention.

FIG. 12a schematically shows a F₂ laser system with a monitor gratingand array detector in accord with the fourth aspect of the invention.

FIG. 12b schematically shows a F₂ laser system with a monitor gratingand array detector also in accord with the fourth aspect of theinvention.

FIG. 13 shows an energy detector for use with a F₂ laser system inaccord with the fifth aspect of the invention.

FIG. 14a shows a F₂ laser system including a blue or green referencebeam for wavelength calibration in accord with the sixth aspect of theinvention.

FIG. 14b shows a F₂ laser system including a blue or green referencebeam for beam alignment stabilization accord with the sixth aspect ofthe invention.

FIG. 15a shows a discharge chamber for a F₂ laser in accord with aseventh aspect of the invention.

FIG. 15b shows a cross sectional view of the ribs crossing the gas flowof the laser tube of FIG. 9a where the gas flows into the dischargechamber from the gas flow vessel, wherein the ribs are separated byopenings to permit the gas flow and aerodynamically shaped to providemore uniform gas flow and the ribs further serve as low inductivitycurrent return bars.

FIG. 15c shows a cross sectional view of the ribs crossing the gas flowof FIG. 9a separated by openings to permit gas flow from the dischargechamber back into the gas flow vessel, wherein the ribs areaerodynamically shaped and separated by openings through which gas exitsthe discharge chamber and flows back into the gas flow vessel.

FIG. 16a shows a F₂ laser resonator, particularly having Brewsterwindows on the discharge tube, for providing a substantially polarizedoutput beam in accord with the eighth aspect of the invention.

FIG. 16b shows a F₂ laser resonator for providing a substantiallypolarized beam also in accord with the eighth aspect of the invention.

INCORPORATION BY REFERENCE

What follows is a cite list of references each of which is, in additionto those references cited above and below, and including that which isdescribed in the related art description and in the priority section,and the above invention summary, and the abstract below, are herebyincorporated by reference into the detailed description of the preferredembodiment below, as disclosing alternative embodiments of elements orfeatures of the preferred embodiments not otherwise set forth in detailbelow. A single one or a combination of two or more of these referencesmay be consulted to obtain a variation of the preferred embodimentsdescribed in the detailed description below and within the scope of thepresent invention. Further patent, patent application and nonpatentreferences are cited in the written description and are alsoincorporated by reference into the detailed description of the preferredembodiment with the same effect as just described with respect to thefollowing references:

German Utility Model No. 299 07 349 1;

U.S. Pat. Nos. 5,901,163, 5,856,991, 6,028,879, 5,559,816, 4,977,563,4,611,270, 6,061,382, 5,406,571, 5,852,627, 3,609,856, 5,095,4923,471,800, 3,546,622, 5,440,574, 6,014,206, 5,373,515, 6,128,323, and5,479,431;

Japanese patents no. 8-274399, 2-152288, 60-16479, and 62-160783; and

U.S. patent application Ser. Nos. 60/178,445, 09/271,020, 09/317,527,09/317,695, 09/130,277, 09/244,554, 09/454,803, 60/212,183, 09/657,396,09/484,818, 09/599,130, 09/602,184, 09/453,670, 09/629,256, 60/173,993,60/166,967, 60/170,919, 60/200,163, 60/215,933, 60/235,116, 60/140,532and 60/140,531, each of which is assigned to the same assignee as thepresent application;

R. Hultzsch: Gitterprismen, Photonik (September 1998), p. 40;

W. Demtroder: Laser Spectroscopy Springer, Berlin Heidelberg (1996) p.112; and

W. A. Taub: Constant Dispersion Grism Spectrometer for Channeled SpectraJ. Opt. Soc. Am. A7 (1990) p. 1779.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 schematically shows a F₂ laser system in accord with the presentinvention. The system includes a laser chamber 2 filled with a gasmixture and having a pair of main electrodes 3 and one or morepreionization electrodes (not shown). The electrodes 3 are connected toa solid-state pulser module 4. A gas handling module 6 is connected tothe laser chamber 2. A high voltage power supply 8 is connected to thepulser module 4. A laser resonator is shown surrounding the laserchamber and including a rear optics module 10 and a front optics module12. An optics control module 14 communicates with the rear and frontoptics modules 10, 12. A computer or processor 16 control variousaspects of the laser system. A diagnostic module 18 receives a portionof the output beam 20 from a beam splitter 22.

The gas mixture in the laser chamber 2 typically includes about 0.1% F₂and 99.9% buffer gas. The buffer gas preferably comprises neon and maybe a mixture of neon and helium (see the '526 application, mentionedabove). A trace amount of a gas additive such as xenon, argon or kryptonmay be included (see U.S. patent applications Ser. Nos. 09/513,025 and60/160,126, which are each assigned to the same assignee as the presentapplication and are hereby incorporated by reference).

The gas mixture is preferably monitored and controlled using an expertsystem (see the '034 application, mentioned above, and U.S. Pat. No.5,440,578, which is hereby incorporated by reference). One or more beamparameters indicative of the fluorine concentration in the gas mixture,which is subject to depletion, may be monitored, and the gas supplyreplenished accordingly (see the '882, '052, '525, '034, '717 and '062applications, mentioned above). The diagnostic module 18 may include theappropriate monitoring equipment or a detector may be positioned toreceive a beam portion split off from within the laser resonator (seethe '052 and '130 applications, mentioned above; see also U.S. patentapplication No. 60/166,967, which is assigned to the same assignee asthe present application and is hereby incorporated by reference). Theprocessor 16 preferably receives information from the diagnostic module18 concerning the halogen concentration and initiates gas replenishmentaction such as micro-halogen injections, mini and partial gasreplacements, and pressure adjustments by communicating with the gashandling module 6.

Although not shown, the gas handling module 6 has a series of valvesconnected to gas containers external to the laser system. The gashandling module 6 may also include an internal gas supply such as ahalogen and/or xenon supply or generator (see the '025 application). Agas compartment or (not shown) may be included in the gas handlingmodule 6 for precise control of the micro halogen injections (see the'882 and '717 applications, mentioned above, and U.S. Pat. No.5,396,514, which is assigned to the same assignee as the presentapplication and is hereby incorporated by reference).

The wavelength and bandwidth of the output beam 20 are also preferablymonitored and controlled. A preferred wavelength calibration apparatusand procedure are described at the '344 application, mentioned above,and at U.S. Pat. No. 4,905,243, which is hereby incorporated byreference. The monitoring equipment may be included in the diagnosticmodule 18 or the system may be configured to outcouple a beam portionelsewhere such as from the rear optics module, since only a smallintensity beam portion is typically used for wavelength calibration (seethe '344 application). The diagnostic module 18 may be integrated withthe front optics module 12, and the line-narrowing components of theresonator may be integrated in the front optics module 12, as well, suchthat only a HR mirror and an optional aperture are included in the rearoptics module 10 (see U.S. patent application no. not yet assigned, ofDr. Klaus Vogler and Dr. Uwe Stamm, entitled, “Resonator for Single LineSelection”, filed Nov. 22, 2000, which is assigned to the same assigneeas the present application and is hereby incorporated by reference).

Preferred main electrodes 3 are described at U.S. patent applicationSer. Nos. 60/128,227, 09/453,670 and 60/184,705, which are each assignedto the same assignee as the present application and are herebyincorporated by reference. Other electrode configurations are set forthat U.S. Pat. Nos. 5,729,565 and 4,860,300, each of which is assigned tothe same assignee and is hereby incorporated by reference. Preferredpreionization units are set forth at U.S. patent application Ser. Nos.60,162,845, 60/160,182 and 09/247,887, each of which is assigned to thesame assignee as the present application and is hereby incorporated byreference. The preferred solid state pulser module 4 and the highvoltage power supply 8 are set forth at U.S. Pat. No. 6,020,723 and U.S.patent application Ser. Nos. 09/432,348, 08/842,578, 08/822,451,60/149,392 and 09/390,146, each of which is assigned to the sameassignee as the present application and is hereby incorporated byreference into the present application.

The resonator includes optics for line-selection and also preferably fornarrowing the selected line (see U.S. patent application Ser. Nos.09/317,695, 09/317,527, 09/657,396, 60/212,183, 09/599,130, 60/170,342,60/166,967, 60/170,919, 09/584,420, 60/212,257, 60/212,301, 60/215,933,09/130,277, 09/244,554, 60/124,241, 60/140,532, 60/140,531, 60/147,219,and 09/073,070, setting forth preferred line selection other than or inaddition to that set forth in accord with the present invention, below,and U.S. Pat. Nos. 5,761,236 and 5,946,337, each of which is assigned tothe same assignee as the present application, and U.S. Pat. Nos.5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991, 5,898,725,5,901,163, 5,917,849, 5,970,082, 5,404,366, 4,975,919, 5,142,543,5,596,596, 5,802,094, 4,856,018, and 4,829,536, all of which are herebyincorporated by reference). Some of the line selection and/or linenarrowing techniques set forth in these patents and patent applicationsmay be used in combination with or alternative to any of the aspects ofthe invention set forth below.

Also particularly for the molecular fluorine laser system, an enclosure(not shown) may seal the beam path of the beam 20 such as to keep thebeam path free of photoabsorbing species. Smaller enclosures may sealthe beam path between the chamber 2 and the optics modules 10 and 12.Advantageously, as mentioned below in accord with a preferredembodiment, the diagnostic components may be integrated into the frontoptics module 12, separate enclosure components that would otherwise beused between, e.g., a separate diagnostic module 18 and beam splittermodule 22, or between the front optics module 12 and beam splittermodule 22, would not be used. The preferred enclosure is described indetail in U.S. patent application Ser. Nos. 09/343,333, 09/598,552,09/594,892, 09/131,580 and 60/140,530, each of which is assigned to thesame assignee and is hereby incorporated by reference, and alternativeconfiguration are set forth at U.S. Pat. Nos. 5,559,584, 5,221,823,5,763,855, 5,811,753 and 4,616,908, all of which are hereby incorporatedby reference.

FIG. 3a schematically shows a F₂ laser resonator including atransmission diffraction grating 32 for line selection in accord with afirst embodiment and the first aspect of the invention. The resonatorincludes the laser chamber 2 including electrodes 3, and is filled witha gas mixture, and also preferably includes a heat exchanger and fan forcirculating the gas mixture, among other components not shown but whichare known to those skilled in the art as being included with an excimerlaser tube, such as baffle boxes and a precipitator for keeping thelaser windows clean, preionization, aerodynamic components, etc., ahighly reflective resonator reflector 30, a transmission diffractiongrating 32 and an output coupler 34. The output coupler 34 may bereplaced with a second highly reflective mirror and output coupling maybe performed by polarization outcoupling from a tilted surface of apolarization beam splitter or a prism or other optical element in theresonator. This alternative outcoupling may be used with other aspectsof the invention set forth below, as well.

The transmission grating 32 disperses the beam as it passes through. Asingle line (e.g., λ₁) of the characteristic plural emission lines ofthe F₂ laser (see FIGS. 1a and 1 b) remains within the acceptance angleof the resonator and the other line or lines (e.g., including λ₂) is/areselected out, as illustrated. The grating 32 is aligned at theappropriate angle, as illustrated, so that the selected line λ₁ iscentrally positioned within the acceptance angle of the resonator. Meansfor rotating the grating 32 may be included for performing the alignmentonline or otherwise. Although not shown, optics for narrowing theselected line λ₁ are preferably included in the laser resonator such asan etalon, one or more apertures and/or a prism or prisms (see the '527and '396 applications, mentioned above).

FIG. 3b schematically shows a F₂ laser resonator including atransmission diffraction grating 36 as an output coupler in accord witha second embodiment and the first aspect of the invention. The grating36 a disperses the beam 20 such that the beam 20 including only theselected line λ₁ is used for industrial processing applications forwhich the laser is intended. Advantageously, visible (red) atomicfluorine emission is also filtered from the beam at the gratingoutcoupler 36 a. The grating 36 a also reflects a portion of the beamback into the gain medium. As such, the grating 36 a performs the dualfunction of dispersing the beam 20 for line selection and outcouplingthe laser beam 20 (and also filtering the red light).

The grating 36 a may be configured as in FIG. 3b to have a partiallyreflective surface for reflecting light back into the gain medium, whileonly the light transmitted at the partially reflective surface isincident at the ruled or grating surface of the grating 36 a. As such,the beam is dispersed and the primary line λ₁ around 157.62 nm isselected, while the secondary line λ₂ around 157.52 nm is dispersed awayfrom the beam acceptance angle, after the beam is output coupled. Inthis way, the grating 36 a serves as a spectral filter outside the laserresonator. Alternatively, the surface facing the chamber 2 may have anAR coating on it, and the ruled surface of the grating 36 a is partiallyreflective, such that only the primary line λ₁ is reflected back intothe chamber to be amplified by the gas mixture or gain medium.

The grating 36 b is a variation of the latter of the embodiments justdescribed relating to the grating 36 a. The grating 36 b shown in FIG.3c has a ruled or grating surface facing the discharge chamber 2. Inthis way, the unselected line λ₂ is dispersed away from the acceptanceangle of the resonator and only the selected primary line λ₁ isreflected back into the chamber 2 for amplification by the gain medium.Preferably, the back surface away from the chamber 2 of the transmissiongrating 36 b has an antireflective (AR) coating formed on it to reduceor prevent reflection from the back surface.

In the second embodiment, one optical element serves at least twofunctions, advantageously reducing the number of potentially lossysurfaces the beam encounters and reducing the overall size of theresonator. In addition, fewer optical components means greatersimplicity for alignment and fewer optical components that may need tobe replaced due to aging. Also, fewer surfaces means less wavefrontdistortions due to imperfections at those surfaces resulting inundesirable bandwidth broadening.

In either of the embodiments in accord with the first aspect of theinvention shown in FIGS. 3a and 3 b, the intensity of the unselectedline λ₂ may be monitored by an energy detector positioned to receive theunselected line λ₂ after having been dispersed away from the acceptanceangle of the beam that includes the primary line λ₁. The detectedintensity of the unselected line λ₂ can provide information about theintensity of the selected line λ₁ or about the gas mixture or laseroptics. Advantageously, with this feature, the beam including theselected line λ₁ does not encounter a beam splitter for reflecting aportion to an energy detector, because the unselected line is used forthis purpose. A portion of the selected line λ₁ may be otherwise splitoff for another purpose such as for monitoring a spectral distributionof the beam 20, or the selected line λ₁ of the main beam 20 may beundisturbed on its way to application processing, while diagnostics areentirely performed using the unselected line λ₂. In either of the aboveembodiments, the material of the grating is at least substantiallytransparent to 157 nm radiation, and as such, calcium fluoride, bariumfluoride, magnesium fluoride, strontium fluoride and lithium fluoride.In other embodiments described herein, transmissive optical componentsare preferably formed from one or more of these materials, and in someembodiments, it may be desired not to use magnesium fluoride due to itsbirefringent nature. In some cases, however, the birefringence ofmagnesium fluoride may be advantageously used (see U.S. patentapplication Ser. Nos. 60/212,257 and 09/317,695, each of which isassigned to the same assignee and is hereby incorporated by reference).

FIG. 4a schematically shows a F₂ laser resonator including a grism 38for line selection in accord with a third embodiment and a second aspectof the invention. The resonator includes a grism 38 and an outcoupler 34on either side surrounding the laser chamber 2. The grism 38 is anintegrated combination of a prism and a grating and advantageouslyprovides improved line selection as a result. The grating and prismaspects of the grism 38 each provide dispersion, or the prism may beused solely to expand the beam to reduce divergence and utilize a largersurface area of the grating surface for improved grating performance,such that the dispersion of the grism 38 is enhanced over that of eithera prism or grating alone.

The grating aspect of the grism 38 may serve to select the desired lineλ₁ while the prism aspect of the grism 38 may serve to narrow theselected line. The prism aspect may serve to expand the beam to enhancethe dispersion of the grating aspect, as well. The back surface of thegrism 38 shown in FIG. 4a is highly reflective so that the grism 38serves the additional function of a resonator reflector, and theadvantages of reducing the number of optical components set forth aboveare also achieved. The grism 40 may also be oriented so that the gratingsurface is first encountered and the smooth back surface of the prismcomponent provides the highly reflective surface to perform theresonator reflector function of the grism 38.

A HR mirror may be included after the grism, wherein no reflectivecoating would be applied to the grism surfaces, and in this embodiment,AR coating would be preferably applied to the grism surfaces. In eithercase, with or without the HR mirror, the entrance surface of the grismclosest to the laser tube 2 preferably includes an AR coating thereon.

FIG. 4b schematically shows a F₂ laser resonator including a grism 40 asan output coupler in accord with a fourth embodiment and the secondaspect of the invention. The grism 40 shown at FIG. 4b serves themultiple functions of line selection, line narrowing and outputcoupling, with advantageous reduction in the number of opticalcomponents typically used for providing all of these functions. Thegrism 40 preferably has a partially reflecting surface at the surface ofthe prism component away from the grating to perform the function of anoutput coupling resonator reflector. The grism 40 may also be orientedso that the grating surface provides the partially reflecting surfaceand the beam ultimately exits at the smooth prism side of the grism 40.Also, the grism 40 may be oriented as in FIG. 4b, and differ from thepreferred arrangement described above in that an AR coating may beapplied to the smooth prism surface closest to the discharge chamber 2,and the grating surface may serve as the partially reflecting resonatorreflector surface. In any of these alternative configurations,advantageously, red atomic fluorine emission is also filtered at thegrism outcoupler 40, and the unselected line (e.g., the secondary linearound 157.52 nm) may be used for diagnostic purposes as described abovewith respect to FIG. 3b.

The grating portion of either of the grisms 38, 40 set forth above maybe on the surface of the prism material or may be etched into the prismmaterial such as by ion beam etching (see U.S. patent application No.60/167,835, which is assigned to the same assignee and is herebyincorporated by reference). These two grism configurations are discussedby R. Hultzsch, “Gitterprismen”, Photonik, p. 40 (September 1998), whichis hereby incorporated by reference. Additional grism discussions areset forth at the U.S. patent application Ser. Nos. 09/602,184 and60/140,532, assigned to the same assignee as the present application,and U.S. Pat. Nos. 5,625,499 and 5,652,681, each of which is herebyincorporated by reference.

The grism 40 as an output coupler is also preferably used with a KrFexcimer laser and with an ArF excimer laser. In order to achieve analternative resonator configuration without the grism 40 with comparabledispersive power to the resonator having the grism 40 as an outputcoupler according to a preferred embodiment, a very large angle prism,two or more prisms or a high dispersive grating may be used. However,all of these alternatives introduce high losses for the laser radiationbouncing back and forth within the resonator. This tends to worsen theratio between broadband background radiation and the selected line ornarrow band emission. In addition, to perform line selection or linenarrowing with sufficient angular dispersion, the resonator would bebent or elongated. This causes additional power losses. Besides thedispersive elements, resonator mirrors and beam steering mirrors wouldlikely be used. This makes resonator alignment using a pilot orreference laser more difficult.

A brief discussion of dispersion by an ordinary prism is discussed hereto illustrate the advantages of using the grism 40 as an output coupleror otherwise in a preferred excimer or molecular fluorine laser which isdescribed further below. FIG. 5 shows a dispersive prism 101illustrating its angular dispersive properties. In FIG. 5, the followingreference characters are used:

ε: prism apex angle

α₁: angle of incidence

α₁: output reflection angle

a: beam dimension

n: refractive index of the prism

L: length of the basis

Θ: angle of beam deflection.

An incident chief ray 102 is shown in bold while outer or marginal raysof an incident laser beam are shown by dashed lines. The incident chiefray impinges upon an incidence surface 103 of the prism 101 at anincident angle α₁ whereupon the ray 102 is refracted based on theSnell's relation. Since, the index of refraction n of the prism materialis wavelength dependent, the angle of refraction differs depending onthe wavelength producing dispersion of the wavelengths of the incidentray 102. The refracted ray 104 is then incident at an exit surface 105of the prism 101 and is again refracted at an angle depending on itswavelength producing an exit ray 105 deviated from its incidentdirection, or the direction of the incident ray 102 by an angle Θ. Forminimum deviation:

α₁=α₂=α;

and

Θ=2α−ε

The angular dispersion of the incident beam illustrated by the chief ray102 is given by:

dΘ/dλ=(dΘ/dn)(dn/dλ)  (a),

and

dΘ/dλ=[2 sin(ε/2)/(1−n ² sin²(ε/2))^(½)](dn/dλ)  (b)

FIGS. 6a and 6 b ilustrate two alternatively preferred prism-grating orgrism configurations. FIG. 6a shows a grism wherein the grating isattached to the back face of a prism. FIG. 6b shows a grism wherein thegrating is engraved or etched into the solid material.

The material of the grisms of either FIG. 6a or 6 b may be glass, fusedsilica, CaF₂, BaF₂, MgF₂ or another similarly transmissive opticalmaterial, adapted to the special wavelength of interest. For excimerlasers and especially the molecular fluorine laser (F₂-laser) emittingnear 157 nm, CaF₂, BaF₂, MgF₂, and LiF, are possible materials, whereCaF₂ is the preferred material for λ<200 nm such as for ArF lasersemitting around 193 nm and for the F₂ laser emitting around 157 nm.

FIGS. 7a-7 b shows a comparison of actions of a reflective grating (FIG.7a) and a grism (FIG. 7b) as wavelength selective retroreflectors. Forthe reflective Littrow grating of FIG. 7a:

α=blaze angle; and

Θ=angle of incidence

A maximum reflectivity is achieved for wavelengths satisfying thereflection relation:

α=Θ;

where,

λ₀=(2d/m)sin Θ  (4)

d=grating constant

m=diffraction order.

For a reflective grism, as in FIG. 7b, where an incident ray isrefracted at an incidence surface 108, propagates through the prismmaterial 110 of the grism, and reflects from a back grating surface 112of the grism, a maximum reflectivity is also achieved at a certainwavelength λ₀. Moreover, a bandwidth of the retroreflected beam isreduced by the dispersive powers of both the prism 110 and the grating112 components of the grism, such that a spectral range that remainswithin the acceptance angle of the resonator upon reflection is greatlyreduced. That is, the dispersion from the grism is a combined actionbetween a pre-dispersion produced by a prism 110 (including beamexpansion of the spectral range not dispersed from the acceptance angleof the resonator) and the grating surface 112.

FIG. 8 schematically illustrates a grating-prism (grism) including aprism 114 and an attached or etched in grating 116 designed to achieve astraight-through chief ray path for a selected wavelength. As shown inFIG. 8:

φ=prism apex angle;

Θ=blaze angle of the grating 116 (wherein, note that the grating 116 maybe attached (as shown) or cut or etched into the original prism 114);

α, β=the angles of incidence and refraction with respect to a normal tothe grating 116, respectively;

n, n_(E), n′=refractive indices of the prism material 114, grating 116at ambient atmosphere and air, respectively, wherein the material of theprism 114 may the same as the material of the grating 116, e.g, CaF₂ maybe used for both, such that n=n_(E).

To achieve a straight through beam path:

α=−β=φ;

and

φ=Θ  (2)

As for the wavelength:

λ₀=(d/m)(n−1)sin φ(2a)

and the angular dispersion may be written as:

dΘ/dλ=m/d(n−1)cos Θ  (3)

dΘ/dλ=(1/λ)tan λ  (3a)

FIGS. 9a-9 c and 9 f schematically illustrates dispersive resonators forline narrowing or line selection that do not include the grism of thispreferred embodiment. FIGS. 9d-9 f and 9 g illustrate the relativerespectively narrowed bandwidths of a beam 205 output from theresonators illustrated at FIGS. 9a-9 c and 9 h.

FIG. 9a shows a semi-narrow band resonator including a discharge chamber201, a dispersive prism 202, a highly reflective mirror 203 and anoutcoupler 204, and including an intracavity aperture 208, for producinga output beam 205. FIG. 9d illustrates a bandwidth BW_(a) and abackground radiation level U_(a) of the output beam 205 of FIG. 9a. Thebackground radiation level U_(a) is not suppressed by the prism 202located on the opposite end of the resonator from the outcoupler 204.The length of the resonator is shown as L_(a).

FIG. 9b shows a narrow band or semi-narrow band resonator including adischarge chamber 201, a pair of dispersive prisms 202 a and 202 b, anHR mirror 203 and an output coupler 204, and including an intracavityaperture 208, for producing a narrow or semi-narrow output beam. FIG. 9eillustrates a bandwidth BW_(b) and a background radiation level U_(b) ofthe output beam 205 of FIG. 9b. Again, the background radiation levelU_(b) is not suppressed by the prisms 202 a, 202 b located on theopposite end of the resonator from the outcoupler 204. The length of theresonator is shown as L_(b).

FIG. 9c shows a narrow band resonator including a discharge chamber 201,a dispersive prism or a beam expanding prism 212 (a prism beam expandermay include more than one prism), and an outcoupler 204, and includingan intracavity aperture 208, for producing a narrow-band output beam205. FIG. 9f illustrates a bandwidth BW_(c) and a background radiationlevel U_(c) of the output beam 205 of FIG. 9c. Again, the backgroundradiation level U_(c) is not suppressed by the prism or prisms 212 a,nor the grating 206 each located on the opposite end of the resonatorfrom the outcoupler 204. The length of the resonator is shown as L_(c).

FIG. 9g shows a semi-narrow band resonator including a discharge chamber201, a dispersive prism 202, an HR mirror 203 and an outcoupler 204, andincluding an intracavity aperture, for producing a semi-narrow-bandoutput beam 205. FIG. 9h illustrates a bandwidth BW_(d) and a backgroundradiation level U_(d) of the output beam 205 of FIG. 9g. In this case,the background radiation level U_(d) is substantially reduced, and isnearly zero, due to its being dispersed by the prism 202 before passingthrough the output coupler 204. The prism 202 is advantageously disposedon the output coupling end of the resonator to achieve this suppressionof the background radiation. This feature is described in more detailbelow with reference to FIG. 11. The length of the resonator is shown asL_(d).

Also shown in FIG. 9g are a pair of steering mirrors 207 a and 207 b.The prism 202 bends the beam, as shown. The steering mirrors arearranged to bring the beam 205 back to parallel or possibly coaxial withthe intracavity beam direction to the left of the prism 202 in FIG. 9g.

FIG. 10a shows a narrow band resonator including a discharge chamber201, a grism outcoupler 210, an HR mirror 203 and a pair of intracavityapertures 208 and 218 for producing a narrow-band output beam 205. FIG.10b illustrates a bandwidth BW_(d) and a background radiation levelU_(d) of the output beam 205 of FIG. 10a. In this case, the backgroundradiation level U_(G) is substantially reduced, and is nearly zero, dueto its being dispersed by the grism output coupler 210 as it isoutcoupled from the laser resonator. It is preferred that the gratingsurface 220 of the grism 210 be partially reflecting such that the beamis dispersed before travelling back through the discharge chamber 201for further amplification. The grism 210 is advantageously disposed onthe output coupling end of the resonator to achieve this suppression ofthe background radiation.

In addition, the presence of the grating surface 220 of the grism 210,which is not present in the embodiment of FIG. 9g, advantageouslyproduces narrow-band line-selection, and not merely semi-narrow bandline selection. This is particularly advantageous when the laser is abroadband emitter such as an ArF or KrF laser. Improved line-selectionof a single line of multiple lines around 157 nm and suppression of thevisible emission of the molecular fluorine laser are also achieved. Thelength of the resonator is shown as L_(G).

The grism 210 may be rotated for tuning the line-narrowed laser. In thiscase, the straight ahead beam propagation feature may be somewhataltered, but the beam propagation would not be bent substantially, suchthat the beam propagation would remain substantially or significantlystraight, yielding an advantage over purely bent resonatorconfigurations.

Some advantages are clearly realized with the resonator configuration ofFIG. 10a. For the resonator lengths L_(a)-L_(c) and L_(G) of theresonators of FIGS. 9a-9 c and FIG. 10a, below, the following relationis observed (using identical or substantially similar components, e.g.,discharge chamber 201 and so on):

L _(G) <<L _(a) ≈L _(b) ≦L _(c)  (4)

The shorter resonator length can be used to achieve a more compactlaser. Losses are also reduced with the shorter resonator, which isparticularly advantageous for the molecular fluorine laser.

There is also the advantage of reducing the spectral bandwidth:

BW _(a) ≈BW _(d) >BW _(b) ≈BW _(G) >BW _(c)  (5)

An additional prism or other optic may be disposed before the grism 210to bring the degree of line-narrowing achieved with the configuration ofFIG. 10a closer to that achieved with the Littrow grating configurationof FIG. 9c.

The effective suppression of the broadband background radiation:

U _(a) ≈U _(b) ≈U _(c) >>U _(d) ≈U _(G)  (6)

is also favorable for the grism output coupler resonator design.

Considering the properties as compared in the relations (4) to (6)illustrate that the resonator design of FIG. 10a with the grism outputcoupler 210 is a very suitable and convenient solution optimized for atleast semi-narrow band spectral emission. The design of FIG. 10a may beused with additional optics to achieve a narrow band output beam (e.g.,less than 0.6 pm) by inserting one or more additional optics such as aprism before the output coupling grism 210, and while still achieving ashorter resonator.

Considering the dispersive power of the resonator of FIG. 10a, theincrease of the dispersive power of a grism over that provided by onlythe dispersive prism 202, e.g., as shown in FIG. 9g, to a value similarto that provided by the high dispersive grating in Littrow mount shownin FIG. 9c can be demonstrated by using the relevant equations andcalculating the angular dispersion.

The greater the angular dispersion, the stronger is the dispersivepower. Therefore, the effect of line narrowing or efficiency of lineselection (or suppression of a second line nearby the selected one forthe molecular fluorine laser) is advantageously improved.

For otherwise identical values such as prism apex angle, refractiveindex and angle of incidence, the angular dispersion, e.g., for theprimary line λ=157.6299 nm of the multiple lines around 157 nm for themolecular fluorine laser (wherein a single element is used in each case)is estimated as follows, wherein CaF₂ is assumed to be the refractivematerial:

Beginning with using equation (1), above, from the discussion relatingto FIG. 5, dΘ/dλ for the prism 202 is calculated as

dΘ/dλ=2.48×10⁻³ mrad/pm.

For CaF₂,

n (at 157.63 nm)=1.5587, and dn/dλ(157 nm)=−0.002605/nm and the prismapex angle ε−45°, and the angle of incidence is equivalent to the blazeangle as used above.

For a Littrow grating, using equation (4),

 dΘ/dλ=6.34×10⁻³ mrad/pm, and λ=(2d/m)sin Θ, as usual.

For a grism, using equation (3),

dΘ/dλ=6.34×10⁻³ mrad/pm;

dλ/dΘ=(2d/m)cos Θ=λ/sin Θ;

and

cos Θ=λ/tan Θ

wherein, λ₀=157.6299 nm is the selected wavelength, Θ=45° is the angleof incidence. In addition, with CaF₂ being the same materials describedabove, and φ=Θ such that the prism angle φ of FIG. 8 is equal to theblaxe angle Θ, a straight through beam path is realized through thegrism output coupler 210 as shown in FIG. 10a.

In brief, using a grism 40 as an output coupler (see FIGS. 10a-10 b)provides at least the following advantages over alternative resonatordesigns, such as those shown at FIGS. 9a-9 c and 9 g:

1. There is an increase of the dispersive power when only one element,i.e., a grism 210, is used, or an increase of the dispersive power ofone element, i.e., the grism 210, of two or more of a line-narrowedresonator.

2. There is a reduction of resonator losses by using only one element,or by combining two elements in one, wherein fewer lossy opticalinterfaces are within the resonator. The reduced resonator size may alsocontribute to additional reduction in losses by absorption.

3. A very short resonator is provided, due to the presence of the grism210, which combines both line selection and output coupling functions inone element, wherein preferably no coating is used with the grism 210such as may be used with an outcoupling mirror.

4. A straight ahead beam propagation is achieved for the one selectedwavelength which fulfills the straight-through path equation for thegiven grism 210 (see equation 2).

5. Because the grism 210 is located at the output side of the laserresonator, the grism 210 suppresses any parasitic background or secondline emission, which is generated in the last resonator round trip, veryefficiently.

6. Resonator adjustment by a pilot or reference laser is facilitatedwhich does not transmit an oblique prism.

The resonator with the grism output coupler 210 has a same orsubstantially a same dispersive power as a blazed grating in Littrowconfiguration and about a factor of 2.5 higher dispersive power as aprism with the same prism angle. To achieve a comparable dispersion withusual prisms, at least a second prism is used to increase the dispersioneffect by a factor of 2. A similarly compact resonator, however, asachieved with the grism output coupler 210 of FIG. 10a is not presentwith such a multiple prism configuration.

FIG. 11 schematically shows a F₂ laser resonator having line selectionfully performed at the front optics module 12 of the resonator in accordwith the third aspect of the invention. The wavelength selector 40 isschematically illustrated as fully integrated with the front opticsmodule 12 in FIG. 11. The wavelength selector 40 may include any of theline selection techniques discussed herein (see FIGS. 3b and 4 b) or inthe patents and patent applications set forth above. Optics of thewavelength selector may include one or etalons and/or prisms, a grating,a birefringent plate (see the '695 application), a grism, etc. Theresonator is advantageously simplified and may be shortened. Forexample, the highly reflective mirror 30 may be brought closer to thelaser chamber 2 than if additional optics were included with the rearoptics module including the mirror 30. The highly reflective mirror mayeven be a window of the laser chamber 2. Also optics control and beammonitoring can each take place around the front optics module permittingsome versatility in overall laser system and housing design.

The beam is advantageously output coupled on a same side of thedischarge chamber 2 as the line selection is performed in thisembodiment. Preferably, the line-selection occurs prior to outputcoupling, such that radiation emanating directly from the dischargechamber is line-selected and/or line-narrowed prior to being outputcoupled, thus improving spectral purity (for alternative embodimentsaccording to this feature, see U.S. patent application No. 60/166,967,which is assigned to the same assignee and is hereby incorporated byreference). The output coupler itself may perform line selection, e.g.,using an outcoupling prism, grating, grism, birefringent prism orcrystal (see below and U.S. patent application No. 60/212,257, which isassigned to the same assignee and is hereby incorporated by reference),or an output coupling interferometer (see U.S. patent application serialno. not yet assigned, of Kleinschmidt and Lokai, for “Narrow BandExcimer or Molecular Fluorine Laser Having an Output CouplingInterferometer”, filed Nov. 17, 2000, which is assigned to the sameassignee and is hereby incorporated by reference). Diagnostic tools maybe included in a same front optics module with the outputcoupler/line-selection optic or optics, such as any of those shown atFIGS. 6a through 8 b, or other tools for monitoring the pulse energy,beam power, wavelength, bandwidth, spatial or temporal pulse shape,amplified spontaneous emission (ASE), discharge width, breakdownvoltage, other parameters indicative of the fluorine concentration inthe tube, etc.

FIG. 12a schematically shows a F₂ laser system with a monitor grating 44and array detector 46 in accord with a fifth embodiment and the fourthaspect of the invention. The beam 20 is outcoupled from the front opticsmodule 12 and impinges upon a highly reflective or substantiallyreflective mirror 42. The highly reflective mirror typically has areflectivity around 96%. The beam 20 is reflected from the HR mirror 42and continues on to the industrial application for which it wasintended, preferably first being redirected by another HR mirror 47.Although not shown, a reference beam may be provided behind the mirror47 for propagating collinear with the beam 20 for controlling analignment of the beam 20 (see the '967 application).

The beam portion 43 that passes through the HR mirror 42 next encountersa grating 44. In FIG. 12a, the grating 44 is a reflection grating, but aconfiguration using a transmission grating also may be used. Also,another dispersive element such as a prism or grism may be used ratherthan the preferred reflection grating 44, and the grating is preferreddue to its high dispersive power over the prism and its simplicityrelative to the grism. The beam portion 43 is dispersed by the grating44 and the dispersed components of the beam portion 43 are detected atan array detector 46, such as a CCD array 46.

The intensities of each of the selected line λ₁ and the unselected lineλ₂ are separately monitored at the CCD array. If the line selection(shown here performed at the front optics module 12, although any of theabove described techniques or those set forth in any of the patents orpatent applications referred to above may be used in some embodiments)is performed optimally, then the intensity of the unselected line λ₂will be very small, and ideally zero. However, if the intensity of theunselected line is above the intensity that is expected, then thewavelength selector may not be optimally aligned, or a component may notbe performing optimally. Thus, the performance of the wavelengthselector can be advantageously monitored in accord with the fourthaspect of the invention. The divergence of the wavelength selector mayalso be monitored by monitoring the beam profile with an array detector(wherein a grating is not before the detector).

Depending on the intensity information received, the optics of thewavelength selector may be adjusted in a feedback arrangement tominimize the intensity of the unselected line λ₂, or to maximize theratio of the intensities of the selected and unselected lines λ₁/λ₂. Theintensity of the selected line or both lines may be monitored and thedriving voltage may be controlled for stabilizing the energy of the beam20, or the gas mixture may be adjusted to stabilize various beamparameters, based on the intensities detected.

FIG. 12b schematically shows a F₂ laser system with a monitor grating 44and array detector 46 also in accord with a sixth embodiment and thefourth aspect of the invention. In contrast with the arrangement setforth at FIG. 12a, a beam splitter 48 is provided between the laserchamber 2 and the front optics module 12 for reflecting a portion of thebeam toward the monitor grating 44. The beam 20 is advantageouslyoutcoupled directly to its destination. The beam splitter 48 can be apolarizing element (as a Brewster surface or thin film polarizer) toimprove the degree of polarization of the output beam.

FIG. 13 shows an energy detector 49 for use with a F₂ laser system inaccord with a seventh embodiment and the fifth aspect of the invention.A beam splitter 50 redirects a beam portion 51 towards the energydetector 49, allowing the main beam 20 to pass through. The detector 49may be a diode or photomultiplier detector, and may be a diamonddetector such as that set forth in U.S. patent application Ser. Nos.09/512,417 and 60/122,145, which are assigned to the same assignee asthe present application and hereby incorporated by reference. Thedetector 49 is preferably particularly designed to be sensitive at 157nm. Optics for filtering the red emission of the laser may be includedsuch as a dispersive element, holographic beam splitter, dichroicmirror(s), or red light filter before the detector, or otherwise as setforth at U.S. patent application Ser. Nos. 09/598,552 and 60/166,952,assigned to the same assignee and hereby incorporated by reference.

The detector 49 is advantageously enclosed in a sealed enclosure 52. Thesealed enclosure 52 is preferably sealably connected with a beam pathenclosure 53 that encloses the path of the outcoupled main beam 20 andthat is itself sealably connected to the laser resonator such that thebeam 20 is never exposed to and absorbed by oxygen and water in ambientair (see U.S. patent application Ser. No. 09/343,333, which is assignedto the same assignee as the present application and is herebyincorporated by reference). The entire resonator itself is also keptfree of the photoabsorbing species such as by using a pair of smallerenclosures 55 a and 55 b between the laser tube 2 and the rear and frontoptics modules 10 or 30 and 12 or 40, respectively.

Photoabsorbing species such as oxygen, hydrocarbons and water areremoved from the enclosure 52, such as by pumping them out with a highvacuum pump, such as a turbo pump, or by pumping for a long time with arotory or mechanical (roughing) pump. The pumping can be continued untilhigh vacuum is reached. However, preferably only a roughing pump (notshown) is used and a series of pumping steps each followed by purgingwith inert gas are performed more quickly and with better results, suchas is described in the '333 application relating to the beam pathenclosure 53.

After the contaminants are removed, a low flow of inert gas such asargon or helium continuously purges the sealed enclosure while the laseris operating. The enclosure 52 and the enclosure 53 may be open to oneanother such that the same purging gas fills both enclosures 52 and 53,or the enclosures 52 and 53 may be separately maintained. The flow rateof the purging inert gas is selected such that only a slightoverpressure is maintained in the enclosure 52. For example, 1-10 mbaroverpressure is preferred, and up to 200 mbar overpressure could beused. The flow rate may be up to 200 liters/hour, and is preferablybetween ten and fifty liters/hour. The flow rate and pressure in theenclosure are precisely maintained using a pressure regulator,flow-control valves and a pressure gauge.

Advantageously, the slight overpressure, precisely maintained, of thelow flow purge in accord with the fifth aspect of the invention mayprevent the strain on optical surfaces that a high flow, high pressurepurge or a vacuum would produce. Fluctuations of the refractive indexwith pressure in the enclosure may also be reduced in accord with thisfifth aspect. Moreover, turbulences typically observed with high flowpurges are avoided, and the rate of contamination deposition on opticalsurfaces is reduced according to this fifth aspect.

An attenuator 54 is preferably positioned before the detector 49 tocontrol the intensity of the incoming light at the sensitive detector49. The attenuator preferably includes a mesh filter. The attenutor 54may include a coating on the detector 49 such as is set forth at U.S.patent application Ser. No. 09/172,805, which is assigned to the sameassignee as the present application and is hereby incorporated byreference.

FIG. 14a shows a F₂ laser system including a blue or green referencelaser 56 for emitting a blue or green beam 57 for wavelength calibrationin accord with an eighth embodiment and the sixth aspect of theinvention. Wavelength calibration techniques using a reference beam 57and coupling the reference beam 57 with a beam portion 60 of the mainlaser beam 20 into a spectrometer (not shown) are set forth at U.S.patent application Ser. No. 09/271,020, which is assigned to the sameassignee as the present application, and U.S. Pat. No. 5,373,515, eachof which is hereby incorporated by reference. Conventional techniquestypically use the red emission of a He—Ne laser (which has two lines at633 nm and 543 nm) for performing this wavelength calibration. However,the red emission (around 630-780 nm) of the F₂ laser can hinder thosetechniques in at least two ways. First, it maybe desirable to reflectout or otherwise filter the red emission from the F₂ laser from the mainbeam portion 60 to improve the spectrometric performance. Second, it maybe difficult to resolve a red reference beam from the red emission ofthe F₂ laser during the spectrometry.

For each of these reasons, a blue or green reference beam 57 is usedadvantageously in accord with the present invention. A solid-state diodelaser that emits blue or green light (e.g., below 550 nm, and preferablybelow 500 nm) is preferably used to generate the reference beam 57. Thered emission from the F₂ laser can then be filtered or reflected outfrom the main beam portion 60 without affecting the reference beam 57.Also, the blue or green light (below 550 nm) of the reference beam 57can be easily resolved from the red emission (630 nm and above) of theF₂ laser.

FIG. 14b shows a F₂ laser system for beam alignment stabilization inaccord with a ninth embodiment and the sixth aspect of the invention. Areference beam 57 is emitted from a blue or green laser, such as a solidstate diode laser, and the main beam 20 is redirected by the reflectors62 and 64 to be collinear with the reference beam 57. Other methods arepossible and understood by persons of ordinary skill in the art. Thealignment of the main beam 20 is stabilized using the reference beam 57as a beam guide. Advantageously, the red emission doesn't disturb theuse of the reference beam, as discussed above with respect to FIG. 14a.Other beam alignment techniques that may be used with the F₂ lasersystem of the present invention are described at U.S. Pat. No.6,014,206, which is assigned to the same assignee as the presentapplication and is hereby incorporated by reference.

FIGS. 15a-15 c illustrate a discharge chamber for a F₂ laser in accordwith a tenth embodiment and the seventh aspect of the invention. Asnoted, it is desired to operate the F₂ laser at high repetition rates(e.g., more than 1 kHz, e.g., 2-4 kHz and above). To achieve this, theclearing ratio, or the gas flow rate (v) through the discharge areadivided by the discharge width (d), or v/d, has to be improved over thatwhich was sufficient at lower repetition rates (e.g., 600-1000 Hz). Thisis because preferably substantially all of the gas within the dischargevolume at the time of a previous discharge moves out of the dischargevolume and is replaced by fresh gas prior to the next discharge.

So, for a F₂ laser having a preferred repetition rate of 2-4 kHz ormore, the clearing ratio to achieve the just stated object would be2000=v/d, or a value twice as large as for a F₂ laser having arepetition rate of 1 kHz. Thus, either the gas flow rate v may beincreased (without enhanced turbulence) or the discharge width d may bereduced to increase the clearing ratio. Both of these are achieved inaccord with the tenth embodiment. Preferably, the tenth embodimentincorporates the discharge chamber design and electrode configurationset forth at U.S. patent application Ser. Nos. 09/453,670 and60/128,227, which are assigned to the same assignee as the presentinvention, and which are hereby incorporated by reference. Some of thepreferred details are set forth below and shown in FIGS. 15a-15 c, andalternative embodiments are described in the '670 application.

FIG. 15a illustrates the tenth embodiment relating to the shape of themain discharge electrodes 68 and 70, and the design of the dischargechamber 2 itself. The shapes of the discharge electrodes 68 and 70significantly effect characteristics of the discharge area 72, includingthe discharge width d. Therefore, at least one, and preferably both, ofthe electrodes 68 and 70 includes two regions. One of these regions, thecenter portion 74, substantially carries the discharge current andprovides a uniform and narrow gas discharge width. The other region, orbase portion 76, preferably in collaboration with other conductive anddielectric elements within the discharge chamber, creates preferredelectrical field conditions in and around the discharge area 72 and alsocontributes to the smoothness and uniformity of the gas flow in thevicinity of the discharge electrodes 68 and 70.

The center portions 74 and base portions 76 preferably form electrode 68and 70 each having a single unit construction, and composed of a singlematerial. The center and base portions 74 and 76 may also comprisedifferent materials, but the different materials should have compatiblemechanical and thermal properties such that mechanical stability andelectrical conductivity therebetween is sufficiently maintained. Thecenter portion 74 and the base portion 76 come together at adiscontinuity or irregularity in the shape of the electrodes 68 and 70.A significant deviation of the electrical field occurs at the locationof the irregularity in such a way that gas discharge occurssubstantially from/to the center portions 74 drastically reducing thedischarge width.

The center portions 74 are shaped to provide a uniform gas dischargehaving a narrow width. The base portions 76 may be shaped according toany of a variety of smooth curves or a combination of several smoothcurves including those described by circular, elliptical, parabolic, orhyperbolic functions. The curvatures of the base portions 76 may be thesame or different, and have the same direction of curvature with respectto the discharge area 72, i.e., the base portions 76 each curve awayfrom the discharge area 72 away from the center portion 74.Alternatively, the base portion 76 of the high voltage main electrode 70may have opposite curvature to the base portion 76 of the electrode 68.That is, the base portion 76 of the electrode 70 may curve toward thedischarge area 72, while the base portion 76 of the electrode 68 curvesaway from the discharge area 60. The alternative configuration providesan even more aerodynamic channel for gas flow through the discharge area72 because the electrode shapes both conform with the shape of the gasflow.

The electrodes 68 and 70 may alternatively have a regular shape and nodiscontinuity between base and center portions 74 and 76. The shape ofthe center portions 74 of the electrodes 68 and 70 in this alternativeconfiguration is preferably similar to that described above and shown.However, the base portions 76 taper to the center portions in atriangular shape where the apexes of the triangular shaped basedportions 76 are the center portions and are rounded as described above.

FIG. 15a also shows a pair of preferred spoilers 80 in accord with thetenth embodiment. The spoilers 80 are preferably integrated with thechamber at the dielectric insulators 82 on either side of the dischargearea 72. The spoilers 80 may be integrated parts of a single unit,single material dielectric assembly with the insulators 82, or they maycomprise different materials suited each to their particular functions.That is, the spoilers 80 and the dielectric insulators 82 may be formedtogether of a common material such as ceramic to provide an aerodynamiclaser chamber 2 for improved gas flow uniformity. Alternatively, thespoilers 80 may be attached to the insulating members 82.

The spoilers 80 are aerodynamically shaped and positioned for uniformgas flow as the gas flows through the chamber 2 from the gas flow vessel84 (partially shown), through the discharge area 72 and back into thegas flow vessel 84. Preferably, the spoilers 80 are symmetric in accordwith a symmetric discharge chamber design.

One end 86 of each of the spoilers 80 is preferably positioned to shielda preionization unit 88 from the main electrode 68, and is shown in FIG.15a extending underneath one of the pre-ionization units 88 between thepreionization unit 88 and the main electrode 68. These ends 86 of thespoilers 80 are preferably positioned close to the preionization units88. For example, the ends 86 may be just a few millimeters from thepreionization units 88. By shielding the preionization units 88 from themain electrode 68, arcing or dielectric breakdown between thepreionization units 88 and the main electrode 68 is prevented. Thespoilers 80 serve to remove gas turbulence zones present in conventionaldischarge unit electrode chambers which occur due to the sharp curvatureof the gas flow in the vicinity of the preionization units 88 and of thegrounded discharge electrode 68.

FIGS. 15b-15 c illustrate another feature in accord with the seventhaspect of the invention. As discussed above, the dielectric insulators82 of the electrode chamber isolate the high voltage main electrode 70.The gas flow is crossed by a first rib configuration 92 a where the gasflow enters the electrode chamber 2 from the gas flow vessel 84 and by asecond rib configuration 92 b where the gas flow exits the electrodechamber 2 and returns the gas back into the gas flow vessel 84. The ribs94 a, 94 b, which are current return bars, are separated by openings forthe laser gas to flow into and out of the electrode chamber 2 from/tothe gas flow vessel 84. The ribs 94 a, 94 b are preferably rigid andconducting, and are connected to the grounded main discharge electrode68 to provide a low inductivity current return path. The conducting ribs94 a of the rib configuration 92 a are preferably substantially shapedas shown in FIG. 15b. The conducting ribs 94 b of the rib configuration92 b are preferably substantially shaped as shown in FIG. 15c. The ribs94 a and 94 b of the rib configurations 92 a and 92 b, respectively, areasymmetrically shaped.

FIG. 15b is a cross sectional view A—A of the rib configuration 92 athrough which the laser gas enters the electrode chamber 2 from the gasflow vessel 84. The ribs 94 a of the rib configuration 92 a each have awide upstream end which meets the laser gas as it flows from the gasflow vessel 84, and a narrow downstream end past which the laser gasflows as it enters the discharge chamber. Preferably, the ribs 94 a aresmoothly tapered, e.g., like an airplane wing, from the wide, upstreamend to the narrow, downstream end to improve gas flow past the ribconfiguration 92 a.

FIG. 15c is a cross sectional view of the rib configuration 92 b throughwhich the laser gas exits the electrode chamber 2 and flows back intothe gas flow vessel 84. The ribs 94 b of the rib configuration 92 b eachhave a wide upstream end which meets the laser gas as it flows from theelectrode chamber 2, and a narrow downstream end past which the lasergas flows as it enters the gas flow vessel 84. Preferably, the ribs 94 bare smoothly tapered, e.g., like an airplane wing, from the wide,upstream end to the narrow, downstream end to improve gas flow past therib configuration 92 b.

The aerodynamic ribs 94 a and 94 b each provide a reduced aerodynamicresistance to the flowing gas from that provided by conventionalrectangular ribs. Together, the effect aerodynamic spoilers 80 and theaerodynamic ribs 94 a and 94 b permit the flow rate of the gas throughthe chamber 2 to be increased without excessive turbulence. Theincreased gas flow rate through the discharge area 72, together with thereduced discharge width provided by the advantageous design of theelectrodes 68 and 70, results in an increased clearing ratio in accordwith high repetition rates of operation of the F₂ laser of the presentinvention.

FIG. 16a shows a F₂ laser resonator for providing a substantiallypolarized output beam in accord with the eighth aspect of the invention.First, Brewster windows 95 are preferably provided on the laser chamber2 ideally exhibiting 100% transmission of π-polarized light and having alower transmissivity of ρ-polarized light. As discussed, for laserswherein the beam undergoes a large number of roundtrips, this effect ofusing Brewster windows substantially serves to polarize the beam.However, for the F₂ laser wherein only 1-2 roundtrips occur, thepolarization is not as high as desired. Thus, other optical elements maybe aligned at Brewster's angle such as prisms, etalons, grisms, etc. forhigher polarization. In addition, a thin film polarization plate 96 isshown in FIG. 16a for providing the desired polarization, e.g., above98%.

FIG. 16b shows a F₂ laser resonator for providing a substantiallypolarized beam also in accord with the eighth aspect of the invention.FIG. 16b shows laser resonator including a double refraction crystal orprism 98 for polarizing the beam in accord with the eighth aspect of theinvention. As shown at FIG. 16b, a double refracting crystal 98comprising a birefringent material such as MgF₂ is used to refract onepolarization component out of the resonator. In this regard, alternativeconfigurations may be found at U.S. patent application No. 60/212,257,which is assigned to the same assignee as the present application and ishereby incorporated by reference. The double refracting crystal may beused as an output coupler, as well. Also, the double refracting crystalmay have beam entrance and/or exit surfaces aligned at Brewster's angleto the beam for additionally improved polarization performance.

The objects of the invention have been met as described above withregard to the first through eighth aspects of the invention. Anefficient F₂ laser for industrial, commercial and scientificapplications such as photolithography and other materials processingapplications has been generally described. Improved line-selectiontechniques for the F₂ laser have been set forth in the first throughthird aspects of the invention. Techniques for monitoring the quality ofthe line selection being performed was set forth in the fourth aspect ofthe invention. An energy detector for use with the F₂ was described asthe fifth aspect of the invention. Techniques for reducing the influenceof the visible emission on the performance of the F₂ laser have beenshown and described according to the sixth aspect of the invention. Alaser chamber for operating a F₂ laser at high repetition rates has beenset forth in the seventh aspect of the invention. Finally, A F₂ laserthat emits a substantially polarized beam, e.g., such that the beamexhibits a 98% or greater polarization has been describe with respect tothe eighth aspect of the invention.

While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat that the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the arts without departing from the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof.

In addition, in the method claims that follow, the operations have beenordered in selected typographical sequences. However, the sequences havebeen selected and so ordered for typographical convenience and are notintended to imply any particular order for performing the operations,except for those claims wherein a particular ordering of steps isexpressly set forth or understood by one of ordinary skill in the art asbeing necessary.

What is claimed is:
 1. A F₂ laser, comprising: a discharge chamberfilled with a laser gas mixture including molecular fluorine forgenerating a spectral emission including a plurality of closely-spacedlines in a wavelength range between 157 nm and 158 nm, said plurality ofclosely spaced lines including a primary line centered around 157.62 nmand a second line centered around 157.52 nm; a plurality of electrodeswithin the discharge chamber connected with a power supply circuit forenergizing the molecular fluorine; and a laser resonator including aline selection unit located on a same side of the discharge chamber asthe laser beam is outcoupled for selecting the primary line around157.62 nm and suppressing said secondary line around 157.52 nm from anacceptance angle of said beam, such that the F₂ laser emits a singlewavelength laser beam having a narrow spectral bandwidth to provide anarrow band VUV beam, and wherein said line-selection unit is located onthe same side of the discharge chamber as the outcoupled laser beam suchthat radiation emanating directly from the discharge chamber isline-selected by the line-selection unit prior to being output coupledfrom said resonator, said location of said line-selection unit providingincreased effectiveness of the line-selection and improved spectralpurity, since the F₂ laser pulse duration is relatively short whencompared with a pulse of a rare gas-halide excimer laser.
 2. The F₂laser of claim 1, wherein the line-selection unit includes atransmission diffraction grating arranged at a particular orientationfor dispersing the plurality of closely-spaced lines including theprimary and secondary lines such that only the primary line centeredaround 157.62 nm remains within the acceptance angle of the resonatorand the secondary line centered around 157.52 nm is dispersed outsidethe acceptance angle of the resonator, such that the F₂ laser emits asingle wavelength laser beam having a narrow spectral bandwidth toprovide a narrow band VUV beam.
 3. The laser of claim 1 wherein thegrating comprises a material selected from the group of materialsconsisting of calcium fluoride, barium fluorine, strontium fluoride,magnesium fluoride and lithium fluoride.
 4. The laser of claim 1,wherein the grating further output couples the laser beam by reflectinga portion of the beam back into the discharge chamber and by dispersingthe remainder of the beam not reflected back into the discharge chamber,to separate the primary and secondary lines, such that a main beam has aspectral distribution substantially centered around said primary lineand substantially not including said secondary line.
 5. The laser ofclaim 1, wherein the grating further output couples the laser beam bydispersively reflecting a portion of the beam back into the dischargechamber, such that said primary line is amplified within the dischargechamber and the secondary line is dispersed away from an acceptanceangle of the resonator, and by transmitting a main beam having aspectral distribution substantially centered around said primary lineand substantially not including said secondary line.
 6. The F₂ laser ofclaim 1, wherein the line-selection unit includes a grism arranged at aparticular orientation for dispersing the plurality of closely-spacedlines including the primary and secondary lines such that only theprimary line centered around 157.62 nm remains within the acceptanceangle of the resonator and the secondary line centered around 157.52 nmis dispersed outside the acceptance angle of the resonator, such thatthe F₂ laser emits a single wavelength laser beam having a narrowspectral bandwidth to provide a narrow band VUV beam.
 7. The laser ofclaim 6, wherein the grism comprises a material selected from the groupof materials consisting of calcium fluoride, magnesium fluoride, bariumfluorine, strontium fluoride and lithium fluoride.
 8. The laser of claim6, wherein the grism further output couples the laser beam by reflectinga portion of the beam back into the discharge chamber, and by dispersingthe remainder of the beam not reflected back into the discharge chamberto separate the primary and secondary lines, such that a main beam has aspectral distribution substantially centered around said primary lineand substantially not including said secondary line.
 9. The laser ofclaim 6, wherein the grism further output couples the laser beam bydispersively reflecting a portion of the beam back into the dischargechamber, such that said primary line is amplified within the dischargechamber and the secondary line is dispersed away from an acceptanceangle of the resonator, and by transmitting a main beam having aspectral distribution substantially centered around said primary lineand substantially not including said second line.
 10. The laser of claim6, wherein the grism further serves as a highly reflective resonatorreflector by dispersing and reflecting the beam, such that said primaryline is reflected back into the discharge chamber and said secondaryline is dispersed away from an acceptance angle of said beam, such thata main beam ultimately outcoupled from the resonator has a spectraldistribution substantially centered around said primary line andsubstantially not including said secondary line.
 11. The laser of claim10, wherein a same optical element within said line-selection unit bothdisperses and output couples the laser beam.
 12. The laser of claim 11,wherein said beam is dispersed before being outcoupled, such that saidprimary line is reflected back into the discharge chamber and outcoupledwithin a accpetance angle of said beam, while said secondary line isdispersed away from the discharge chamber and the acceptance angle ofsaid beam.
 13. The laser of claim 11, wherein said beam is dispersedafter being outcoupled, such that said primary line remains within anacceptance angle of said outcoupled beam and said secondary line isdispersed away from the acceptance angle of said beam.
 14. The laser ofclaim 11, wherein substantially all of the radiation output coupled fromthe resonator is subject to dispersion at said line-selecting opticalelement.
 15. The laser of claim 14, wherein said line-selecting opticalelement is a transmission grating.
 16. The laser of claim 14, whereinsaid line-selecting optical element is a transmission grism.
 17. Anexcimer laser, comprising: a discharge chamber filled with a laser gasmixture including molecular fluorine, an active rare gas and a buffergas for generating a broadband spectral emission; a plurality ofelectrodes within the discharge chamber connected to a power supplycircuit for energizing the gas mixture; and a laser resonator includinga grism, which has a grating surface and a prism body thereby includingtwo optical element functions within one optical element and reducing anumber of optical surfaces and a size of the resonator so that theresonator is more efficient compared with a laser resonator includingseparate prism and grating optical elements to perform the two opticalelement functions of the grism, the grism being arranged at a particularorientation for dispersing the broadband spectrum such that only aselected spectral portion of the broadband spectrum remains within theacceptance angle of the resonator and outer portions of the broadbandspectrum are dispersed outside the acceptance angle of the resonator,such that the excimer laser emits a narrowband DUV laser beam.
 18. Thelaser of claim 17, wherein the grism comprises a material selected fromthe group of materials consisting of calcium fluoride, magnesiumfluoride, barium fluorine, strontium fluoride and lithium fluoride. 19.The laser of claim 17, wherein the grism further output couples thelaser beam by reflecting a portion of the beam back into the dischargechamber, and by dispersing the remainder of the beam not reflected backinto the discharge chamber to narrow the bandwidth of the beam, suchthat a main beam has a spectral distribution substantially centeredaround said selected spectral portion and substantially not includingsaid outer portions of said broadband emission spectrum of the excimerlaser.
 20. The laser of claim 17, wherein the grism further outputcouples the laser beam by dispersively reflecting a portion of the beamback into the discharge chamber, such that said selected spectralportion is amplified within the discharge chamber and the outer portionsare dispersed away from an acceptance angle of the resonator, and bytransmitting a main beam having a spectral distribution substantiallycentered around said selected spectral portion and substantially notincluding said outer portions of said broadband emission spectrum of theexcimer laser.
 21. The laser of claim 17, wherein the grism furtherserves as a highly reflective resonator reflector by dispersing andreflecting the beam, such that said selected spectral portion isreflected back into the discharge chamber and said outer portions aredispersed away from an acceptance angle of said beam, such that a mainbeam ultimately outcoupled from the resonator has a spectraldistribution substantially centered around said selected spectralportion and substantially not including said outer portions of saidbroadband emission spectrum of the excimer laser.
 22. The laser of claim17, wherein the grating surface of the grism is prepared by ion-beametching.